The present invention relates to a color image forming apparatus such as a color electrophotographic apparatus and a color image forming method using this apparatus.
Digital full-color electrophotography uses the following method. First, R (red), G (green), and B (blue) image signals of an original read from a scanner or R, G, and B image signals obtained through an external interface are color-converted into Y (yellow), M (magenta), and C (cyan). Black generation and color correction are performed to produce signals of four colors Y, M, C, and K (black).
After that, magnification change such as enlargement or reduction is performed, and image area separation is performed to separate a character-line region and a halftone region from each other. Furthermore, spatial filter processing for performing edge emphasis-MTF correction, smoothing-moirxc3xa9 removal, and the like is performed, and xcex3 conversion is performed to obtain a linear relationship between the recording signal intensity and the reproduced image density. Finally, to output the data to a laser or a liquid crystal filter, halftoning is performed by assembling a dot pattern or a line pattern by dithering or error diffusion.
Depending on the type of output device, this method expresses not only binary signals but also multilevel signals by varying the intensity of a laser or varying the pulse width, and can thereby more smoothly express halftone with high resolution. Hence, this method is essential for clear image expression.
A photoreceptor having a charged surface is irradiated with the Y, M, C, and K optical signals generated by a laser or a liquid crystal filter through the image processing as described above. This produces a potential drop and forms an electrostatic latent image. The latent image is visualized by bringing charged toner into contact with the image. Other image visualizing schemes are a method (tandem scheme) which uses separate photoreceptors for four colors Y, M, C, and K and separately develops images of these four colors, and a scheme by which latent images of Y, M, C, and K are sequentially formed and developed by a single photoreceptor and transferred onto an intermediate transfer medium or a paper sheet. After being transferred onto a paper sheet, toner components of the four colors are melted to mix these colors by a fixing device such as a heat roller. A final image is obtained through a series of electrophotographic processes as described above.
Both the digital image processing and analog electrophotographic process described above are factors that determine the final image. In particular, the stability of the electrophotographic process is important for the stability of an image. Throughout life, during continuous copying, and against environmental changes, the same image must be output for the same input optical image for each of the four colors. For this purpose, the apparatus includes, as an image quality maintaining mechanism, a mechanism which varies the surface potential or development bias of a photoreceptor or varies the toner specific density, in accordance with the life counter or with temperature and humidity fluctuations. It is also possible to use a mechanism which actually develops a test pattern on a transfer belt or the like and feeds the result back to the process conditions so that the same image density is maintained.
In this manner, the xcex3 values of the four colors must be adjusted to be constant at any time. Otherwise, the image density changes, or a color shift occurs if the four colors change separately. For example, a bluish image or a yellowish image forms. However, the process conditions such as the surface potential and bias of a photoreceptor and the toner specific density cannot be unlimitedly changed from a physical or image-quality viewpoint. Finally, the physical properties of a developing agent, particularly the stability of the charging characteristics is important. For example, if the charge amount becomes too large, the adhesive force to a carrier abruptly increases, so no sufficient ID can be obtained even when the development contrast potential is raised. On the other hand, if the charge amount becomes too small or if 0 charge or reverse charge increases in a charge amount distribution, fog or toner scattering increases even when the white contrast potential is raised.
Also, when multilevel exposure is performed to improve tone reproduction, the potential on the photoreceptor eventually becomes multilevel and is readily influenced by the xcex3 characteristic of development. Hence, the stability of the characteristics of a developing agent is more and more required.
Color toner generally contains a binder, wax, colorant, charge control agent, and additive. It is basically necessary to use colorless materials except for pigments. As a charge control agent (CCA), not colored azo-based CCAs used in monochromatic toner but colorless CCAs such as a complex of salicylic acid-based zinc, boron complex, quaternary ammonium salt, and resin are extensively used. However, no CCA stable throughout its life and against environmental changes is obtained.
As a colorant, an organic pigment is generally used except for carbon black used as black. Examples of magenta pigments are an azolake pigment, xcex2 naphthol insoluble azo pigment, naphthol AS insoluble azo pigment, pyrazolone pigment, quinacridone pigment, carbazole violet pigment, perillene pigment, and thioindigo pigment. Examples of yellow pigments are a monoazo yellow pigment, benzidine yellow pigment, monoazo yellow lake pigment, benzimidazolone pigment, and condensed azo pigment. Examples of cyan pigments are a phthalocyanine pigment and indanthrene blue pigment.
To improve the color generation and transparency, these pigments and a binder resin are generally subjected to preliminary dispersion called masterbatch. Dispersibility is improved by enhancing dispersion by a kneader such as a three-roll mill which applies a shearing force, by dispersing crude pigments before drying, or by adding a dispersing agent. Dispersion is basically dominated by the chemical affinity between pigments and a binder resin.
The dispersibility of a pigment has influence not only on the transparency but also on the chargeability of toner, since the pigment itself has high chargeability. Generally, if the dispersibility is low, the charge amount distribution widens, and this increases fog and toner scattering. Also, the structure of a pigment has influence on the chargeability and the environmental resistance. To improve the dispersibility of a pigment, a large amount of a polar group such as a carboxyl group can be added to the binder resin. This method is readily achievable when a polyester resin commonly used in color toner is used. However, the charge amount easily decreases in a high-humidity environment or by developing agent stirring throughout life.
As described above, pigments are selected in accordance with the dispersibility to resin, the chargeability, and the tone of color transparency. In particular, the color tone is preferably so selected as to be close to Japan colors as standard colors of three primary colors Y, M, and C. In addition, in accordance with recent regulations for the sake of safety, the use of, e.g., a benzidine yellow pigment is often avoided.
Also, to obtain a color tone having high saturation, the transparency of each toner must be high, and it is necessary to obtain a uniform fixing surface with little graininess in order to suppress diffused reflection on the surface or in the grain boundary. To this end, a low-molecular-weight polyester resin which-readily sharply melts is generally used as a binder. However, the mechanical strength of this low-molecular-weight polyester resin is low, so sufficient life is difficult to ensure.
Toner which easily sharply melts often produces offset because the elasticity during melting lowers. Therefore, the general conventional approach is to use a mechanism which prevents offset by steadily coating a fixing roller with silicone oil. In this method, however, the oil adheres to a fixed printed product. Especially when an image is fixed on an OHP sheet, the oil forms fringe patterns on the image or makes the image sticky during storage. Additionally, an oil supplementing mechanism increases the size of the machine, and periodically supplementing the oil to a tank is cumbersome.
To prevent offset, it was attempted to suppress a lowering of the viscoelasticity at high temperatures by changing the molecular weight distribution of a resin. However, achieving this effect and the transparency or color generation of OHP at the same time was difficult. Therefore, in recent years it is being attempted to improve the offset characteristic such that a device for supplementing oil to a heat roller is unnecessary, while maintaining sharp melting by adding low-melting-point wax to toner.
Even when a soft resin or wax is used as a binder, additives such as silica, titanium oxide, and alumina are generally added in amounts larger than in monochromatic toner, for the purposes of holding an appropriate toner flowability and preventing spent toner or filming. This, however, allows easy chargeability change due to environmental changes and makes it difficult to maintain stable chargeability throughout life.
As described above, stable charging characteristics of full-color toner are difficult to maintain against environment changes or throughout life. Also, since the chargeability changes differently by Y, M, C, and K, changes in the chargeability appear as a change in the color tone of a copied image, a change in the image density, fog, toner scattering, and the like. Although it is being attempted to correct such nonuniform charging characteristics by the process conditions or by the mechanism, the correction has its limits. Additionally, frequent correction and adjustment worsen the response and ease of use of the machine.
It is an object of the present invention to provide an image forming apparatus which maintains stable charging characteristics against environmental changes and throughout life, and can form high-quality images having a good color tone and sufficient image density and having no problems in fog and toner scattering.
It is another object of the present invention to provide a method which maintains stable charging characteristics against environmental changes and throughout life, and forms high-quality images having a good color tone and sufficient image density and having no problems in fog and toner scattering.
According to the first aspect of the present invention, there is provided an image forming apparatus comprising first to fourth image carriers, first to fourth developing devices opposing the first to fourth image carriers in one-to-one correspondence with each other to develop first to fourth electrostatic latent images, formed on the first to fourth image carriers, by using first to fourth developing agents, thereby forming first to fourth developing agent images, first to fourth transfer means, arranged after the first to fourth developing devices in one-to-one correspondence with each other, for transferring the first to fourth developing agent images onto a transfer medium, and a fixing device for fixing the transferred first to fourth developing agent images on the transfer medium, wherein as each of the first to fourth developing agents, one of a yellow developing agent containing a polyester resin having a pre-determined acid value, a zirconium complex of a salicylic acid derivative, and a benzimidazolone pigment as a yellow pigment, a magenta developing agent containing a polyester resin having a pre-determined acid value, a zirconium complex of a salicylic acid derivative, and a naphthol AS insoluble azo pigment as a magenta pigment, a cyan developing agent containing a polyester resin having a pre-determined acid value, a zirconium complex of a salicylic acid derivative, and a phthalocyanine pigment as a cyan pigment, and a black developing agent containing a polyester resin having a pre-determined acid value, a zirconium complex of a salicylic acid derivative, and carbon black as a black pigment is applied.
According to the second aspect of the present invention, there is provided an image forming apparatus comprising an image carrier, first to fourth developing devices opposing the image carrier to develop first to fourth electrostatic latent images, sequentially formed on the image carrier, by using first to fourth developing agents, thereby forming first to fourth developing agent images, transfer means, arranged after the first to fourth developing devices, for transferring the first to fourth developing agent images, and a fixing device for fixing the transferred first to fourth developing agent images on a transfer medium, wherein as each of the first to fourth developing agents, one of a yellow developing agent containing a polyester resin having a pre-determined acid value, a zirconium complex of a salicylic acid derivative, and a benzimidazolone pigment as a yellow pigment, a magenta developing agent containing a polyester resin having a pre-determined acid value, a zirconium complex of a salicylic acid derivative, and a naphthol AS insoluble azo pigment as a magenta pigment, a cyan developing agent containing a polyester resin having a pre-determined acid value, a zirconium complex of a salicylic acid derivative, and a phthalocyanine pigment as a cyan pigment, and a black developing agent containing a polyester resin having a pre-determined acid value, a zirconium complex of a salicylic acid derivative, and carbon black as a black pigment is applied.
According to the third aspect of the present invention, there is provided an image forming method comprising the development/transfer step of developing first to fourth electrostatic latent images, formed on first to fourth image carriers, by using first to fourth developing agents having different colors, thereby forming first to fourth developing agent images, and sequentially transferring the first to fourth developing agent images onto a transfer medium, and the fixing step of fixing the transferred first to fourth developing agent images on the transfer medium, wherein as each of the first to fourth developing agents, one of a yellow developing agent containing a polyester resin having a pre-determined acid value, a zirconium complex of a salicylic acid derivative, and a benzimidazolone pigment as a yellow pigment, a magenta developing agent containing a polyester resin having a pre-determined acid value, a zirconium complex of a salicylic acid derivative, and a naphthol AS insoluble azo pigment as a magenta pigment, a cyan developing agent containing a polyester resin having a pre-determined acid value, a zirconium complex of a salicylic acid derivative, and a phthalocyanine pigment as a cyan pigment, and a black developing agent containing a polyester resin having a pre-determined acid value, a zirconium complex of a salicylic acid derivative, and carbon black as a black pigment is applied.
According to the fourth aspect of the present invention, there is provided an image forming method comprising the development/transfer step of developing and transferring first to fourth electrostatic latent images, sequentially formed on an image carrier, by using first to fourth developing agents having different colors, thereby forming first to fourth developing agent images sequentially on a transfer medium, and the fixing step of fixing the transferred first to fourth developing agent images on the transfer medium, wherein as each of the first to fourth developing agents, one of a yellow developing agent containing a polyester resin having a pre-determined acid value, a zirconium complex of a salicylic acid derivative, and a benzimidazolone pigment as a yellow pigment, a magenta developing agent containing a polyester resin having a pre-determined acid value, a zirconium complex of a salicylic acid derivative, and a naphthol AS insoluble azo pigment as a magenta pigment, a cyan developing agent containing a polyester resin having a pre-determined acid value, a zirconium complex of a salicylic acid derivative, and a phthalocyanine pigment as a cyan pigment, and a black developing agent containing a polyester resin having a pre-determined acid value, a zirconium complex of a salicylic acid derivative, and carbon black as a black pigment is applied.
In the present invention, it is possible to maintain stable charging characteristics against environmental changes and throughout life, and form high-quality images having a good color tone and sufficient image density and having no problems in fog and toner scattering.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.